FIGS. 7(a) and 7(b) show a conventional LED array 10. This LED array has a printed circuit board 1 made of glass epoxy resin on which a printed wiring (not shown) is formed. As seen from FIG. 7(a), a plurality of LED chips 2, 2, . . . are disposed linearly in the longitudinal direction of the LED array. Each LED chip 2 is bonded to the board 1 by conductive adhesive agent such as silver epoxy resin. More specifically, each LED chip 2 has electrode plates at the lower and upper surfaces thereof, and the lower electrode plate is bonded to the board 1 by conductive adhesive agent. The upper electrode plate of each LED chip 2 is connected to the printed wiring by a gold fine wire 2A. The gold wire 2A is bonded by using a wire bonding method or the like. Elongated reflection plates 3 and 3 made of white plastics are mounted facing each other, with the train of LED chips 2, 2, . . . being interposed therebetween. Light beams 4 radiated from the LED chips 2, 2, . . . are directed in the main illumination direction 5 perpendicular to the surface of the board 1. The LED chips 2, 2, . . . and reflection plates 3 and 3 are enclosed with a cover 6 having a lens unit 6a made of transparent plastics. Current limiting chip resistors 7 are fixed to the board 1 at positions outside of and along the cover 6, by using solder. The resistor 7 limits the current flowing to the LED chip 2. The LED chip 2 and resistor 7 are electrically connected by the printed wiring formed on the board 1. The reflection plates and cover 6 are fixedly mounted on the board, by inserting legs 3a, 3a, 6b into holes 1a of the board 1 and forming caulking portions 3b, 6c through thermal caulking, as particularly shown in FIG. 7(b).
FIG. 7(a), mounting holes indicated at 1B are used for mounting the LED array 10 within a casing of a bar code reader or the like.
A light beam 4 of the LED array is arranged to be radiated in the direction (main illumination direction) 5 perpendicular to the surface of the board 1. In other words, the surface of the board 1 is aligned to the plane perpendicular to the optical path of the light beam 4. The board 1 has various components mounted thereon. Accordingly, it is inevitable that the dimension of the board 1 becomes large more or less, essentially resulting in a large dimension of an apparatus which mounts such an LED. Attention should be paid so that the board 1 of the LED array built in an apparatus will not intercept a portion of the optical path. This leads to a large optical system and hence a large dimension of the apparatus.
FIGS. 8 and 9 show examples of apparatuses having a large dimension. FIG. 8 shows a part of a so called contact type sensor. Light beams 4 emanated from LED arrays 10 and 10 illuminate the surface 13 of an original. Reflected light beams 4A are received via a rod lens array (Selfoc Lens, merchandise name) by a CCD 12. This rod lens array 11 outputs an image of an inputted image while maintaining its scale. The contact type sensor shown in FIG. 8, particularly the LED arrays 10, cannot be mounted too near the surface 13 of the original because of the large dimension of the board 1, inevitably resulting in a large dimension of the apparatus.
FIG. 9 shows a part of a so-called lens condensing type sensor. A light beam 4 emanated from an LED array 10 illuminates the surface 13 of an original. The reflected light beam 4A is further reflected by a mirror 15 and applied via a lens 16 to a CCD 12. The lens condensing type sensor of FIG. 9, particularly the LED array 10, is required to be positioned where the optical path is not intercepted, inevitably resulting in a large dimension of the apparatus.
The aperture area of the reflection plate 3 of the conventional LED array 10 shown in FIG. 7 is set considerably large relative to the dimension of the LED chip 2, because of a margin of assembly precision necessary for the apparatus quality. As a result, the light beams 4 emanated from the LED chip and reflected by the reflection plates 3 and 3 include invalid light beams. Namely, as particularly shown in FIG. 10, a light beam 4A emanated from the LED chip 2, reflected by the reflection plate 3, and refracted by the lend unit 6A of the cover 6, does not reach within an effective illumination area 18, and becomes an invalid light beam, so that light from the LED chip 2 cannot be efficiently used.
Furthermore, if the main illumination direction is set to the direction perpendicular to the surface of the board 1, it is not possible to form a reflection surface at the back of the LED chip 2, being unable to expect efficient reflection.
FIGS. 11(a) and 11(b) show another example of a conventional LED array. FIG. 11(a) is a perspective view of this LED array, and FIG. 11(b) is a cross section taken along line A--A of FIG. 11(a). The difference of this example from that shown in FIG. 7 resides in a cylindrical lens 6A which is supported by a pair of reflection plates 3 and 3. The other arrangement is the same as that shown in FIG. 7, and like elements to those shown in FIG. 7 are represented by using identical reference numerals.
FIGS. 12(a) and 12(b) show still another example of a conventional LED array. In this example, a pair of reflection plates 3 and 3 holds a board 1 at its opposite lateral sides, and a pair of support pieces 1A and 1A holds the board 1 and reflection plates 3 and 3 at their opposite longitudinal sides. The pair of reflection plates 3 and 3 holds a lens 6A. The other arrangement is the same as that shown in FIG. 7, and like elements to those shown in FIG. 7 are represented by using identical reference numerals.
Similar to the LED array of FIG. 7, the LED arrays shown in FIGS. 11 and 12 radiate a light beam 4 in the direction perpendicular to the surface of the board 1. Accordingly, these LED arrays are also associated with the problems described with the LED array shown in FIG. 7.
In addition, as the integration degree of LED chips becomes high, the number of current limiting resistors essentially increases, so that the mount area for resistors becomes large.
Conventionally, an led array has used a single current limiting resistor for a plurality of LED chips. However, as the illumination requirements become very strict, each LED chip requires its own current limiting resistor, resulting in an increase of the mount area for current limiting resistors. Furthermore, the printed circuit board is mounted perpendicular to the main illumination direction. Therefore, as the area of the printed circuit board becomes large, it is hard to mount an LED array on a thin apparatus such as a bar code reader.
As described above, the main illumination direction of a light beam emanated from an LED chip of a conventional LED array is generally perpendicular to the board. Therefore, it is difficult to make small the area of an LED array on the surface of the board, and it is impossible to efficiently use a light beam emanated from an LED chip.